Page:Popular Science Monthly Volume 10.djvu/189

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177
MORE CONCERNING MECHANICAL TOOLS.

drawn to follow the fibre, should it lead inward. Suppose, however, that in the progress of the work such a place has been reached as would have so drawn the chisel inward. What will happen? Either the strength of the indrawing fibre will be so great that the workman will be unable to propel the tool, or, if not thus impeded, he must by extra effort separate the fibre and so release the tool. This separation, however, will not be by the process of cutting, but by that of tearing, and shavings so torn off will have left their marks in the roughnesses which attend the tearing asunder of fibrous woods. Thus the tool will defeat the very object for which it was designed.

Now, what is it which so forcibly draws, or tends to draw, the tool downward below the surface of the timber? The forces in operation are the hand of the workman and the tenacity of the fibre. If the tenacity is greater than the power, the workman must stop. That the tool cannot follow the direction of the fibre is clear, because the front part of the wooden sole forbids the penetration; but that it may be brought to a standstill, or must tear off the fibre, is also very clear. The mechanician has therefore to consider how to defeat these tendencies, which, as now sketched, result from a collision between the indrawing strength of the fibre and the power of the man to crosscut the fibre by the tool, or else to tear it asunder and leave the surface rough.

Since the tool, as now contrived, cannot efficiently cross-cut the resisting fibre, and since that fibre has to be removed, the object must be either to prevent such an accumulation of fibres as will stop the progress of the tool, or to destroy the fibre piecemeal as it is operative for hinderance. Both plans have been adopted. A consideration of the former may prove introductory to the latter, which appears in almost all attempts to perfect this tool and its appended contrivance.

As the tool progresses, and the fibres become more and more impeding, it will be clear that a portion of this impediment results from a condensation of the fibre in the mouth of the wooden box. The more numerous the fibres admitted here, the greater will be the condensation. This state of affairs can be partially obviated by a narrowing of the mouth of the plane; such an act, of course, requires that the introduced chisel should enter less deeply into the timber being operated upon. Although thus abated, the cause is not removed, and even if so far abated as to prove no real impediment to the workman, yet the quantity of material removed on each occasion will be so small that the tool becomes one for finishing work only, and not for those various operations to which its present powers enable artisans to apply it.

To be the useful tool it is, the mouth must not be so narrowed, nor the inserted chisel so withdrawn, that the shaving is thus the thinnest